There is known a seatback pocket structure including a back board forming a back side of a seatback, and a pocket forming board made of plastic material and attached to the seatback via a flexible hinge (also called an integral hinge) formed by bending a lower part of the pocket forming board, such that a seatback pocket (may be simply referred to as a pocket hereinafter) for storing small articles is defined between the back board and the pocket forming board (see JP2001-70090A and JP2001-294071A, for example).
In such a seatback pocket structure, the flexible hinge allows the pocket forming board to be tilted rearward relative to the seatback. The rearward tilting of the pocket forming board increases the fore-and-aft width of the upper opening of the pocket, so that an article can be put into or taken out of the pocket easily.
In the following description, a state where the pocket forming board extends substantially parallel to the seatback will be referred to as a closed state of the pocket, while a state where the pocket forming board is tilted rearward relative to the seatback will be referred to as an opened state of the pocket.
The flexible hinge achieves a hinge action by deformation of the plastic material involving plastic deformation, and the flexible hinge undergoes plastic deformation every time the pocket is opened or closed, and therefore, a crack may be caused therein relatively easily during use and it is difficult to achieve sufficient durability. To form the pocket forming board having the flexible hinge, it is necessary to bend a flat board formed by injection molding, and this reduces the productivity. Since the flexible hinge is formed by the bent portion of the pocket forming board, the pocket forming board includes a residual stress at the bent portion, and this also makes it difficult to achieve sufficient durability.